An Access Point (AP) can have two transceivers for wireless local area network (WLAN or wireless LAN) applications, such as in the 2.4 GHz and 5 Ghz frequency bands. Both transmitters use a microwave detector diode to sense radio output power. The diode's output voltage is amplified and applied to an analog to digital converter (ADC).
The ADC is part of the digital feedback control loop that is used for transmit automatic level control (ALC). The ALC adjusts the radio power to maintain a target output power at the transmit antenna port. For example, an AP target output power can range from 1 dBm to 20 dBm.
The detector diode circuit response is centered to fit the range of the output voltages inside the ADC window (0 V to 2 V). With 8 bits of ADC resolution, an 8 mV change is required to change the ADC by one digital value. When the ADC value changes, the digital feedback loop makes the appropriate correction to maintain the target output power level at the antenna port.
However, a problem arises because the detector diode response (output voltage vs. input power) is more sensitive (i.e. has a steeper slope) for high transmit output power compared to low transmit output power levels. For example, a typical response is illustrated in table 1. TABLE-US-00001 TABLE 1 Target Output Power Response Sensitivity (dBm) (mV/dB) -1 4 8 25 11 34 20 120.
TABLE 1Target Output PowerResponse Sensitivity(dBm)(mV/dB)−14825113420120At −1 dBm target output power, the ADC changes by 1 count when the power drifts by 2 dB. Then the digital feedback loop makes a correction. The −1 dBm transmit ALC accuracy is coarse (2 dB) because the receiver sensitivity is low (4 mV/dBm). Table 2 shows transmit ALC accuracy at various target output power levels, which is also graphically illustrated in FIG. 6, line 602.
TABLE 2Target Output PowerALC Accuracy(dBm)(dB)−12.0080.32110.24200.07Therefore, the nonlinear response of the diode sensor in the digital control loop reduces transmit power precision at low output power levels. The accuracy is much better at high target output power levels. However, applications such as radio management and high density (pico cells) require more accuracy at low transmit power levels.
An attempted solution to this problem used a detector diode followed by a total of three operational amplifiers (op amp) and two analog SPDT switches (see U.S. application Ser. No. 10/966,902 filed Nov. 23, 2004 and assigned to Cisco Technology, Inc., the assignee of this application). This first op amp functioned as a differential/buffer amplifier stage. The first SPDT switch was used to route the buffered DC voltage through one of two separate parallel op amp gain paths. One op amp was set for a fixed high gain factor. The other op amp was set for a fixed low gain factor. The second SPDT switch selected the high or low gain output and routed the voltage to the ADC. However, this approach required a lot of extra circuitry, which increased cost. Furthermore, substantial printed circuit board (PCB) real estate was required to implement this solution as well as multiple control line routing.